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Abstract:

A method for tracking a moving object by a tracking device comprises the
following steps: (a) reception by the tracking device of a signal
transmitted by the moving object or objects and comprising a first item
of information of the position of the moving object determined from
information received from a satellite positioning system, (b)
determination of a second item of information of the relative altitude of
the moving object with respect to the tracking device, and the said
second item of information is calculated on the basis of the atmospheric
pressures measured at the altitude of the tracking device and on the
moving object, (c) calculation of the relative position of the moving
object with respect to the tracking means, from the first and second
items of information using a data fusion technique, (d) orientation of
the tracking means towards the relative position calculated in step (c).

Claims:

1. Method for the tracking of at least one moving object by a tracking
device, the tracking device comprising at least one tracking means
mounted in an articulated manner on a fixed support, characterized in
that it comprises a step of calibration of the tracking device comprising
accelerometers, inclinometers or a compass, on the support or the
tracking means, in order to determine the orientation of a reference
peculiar to the support and/or to the tracking means with respect to the
local vertical, or in the local geographic reference system and comprises
at least the following steps: (a) reception by the tracking device of a
signal transmitted by the moving object or objects and comprising a first
item of information of the position of the moving object or objects
determined from information received from a satellite positioning system,
(b) determination of a second item of information of the relative
altitude of the moving object or objects with respect to the tracking
device, and the said second item of information is calculated on the
basis of the atmospheric pressures measured at the altitude of the
tracking device and on the moving object or objects, (c) calculation of
the relative position of the moving object or objects with respect to the
tracking means, from a mixture of the first and second items of
information using a data fusion technique, (d) orientation of the
tracking means towards the relative position calculated in step.

2. Tracking method according to claim 1, characterized in that it also
comprises a step (e) of determination of a third item of information of
position of the tracking device from information received from a
satellite positioning system.

3. Tracking method according to claim 1, characterized in that it also
comprises a step (f) of determination of a fourth item of information of
the distance between the moving object or objects and the tracking
device.

4. Tracking method according to claim 1, characterized in that it also
comprises a step (g) of determination of a fifth item of information of
the angle of arrival of the signal in a horizontal plane.

5. Tracking method according to claim 2, characterized in that it
furthermore comprises in step (c) the third and/or the fourth, and/or the
fifth item of information.

6. Tracking method according to claim 1, characterized in that the
calculation of the relative position of the moving object or objects
takes into account a behavioural model of the moving object or objects by
adding information on the nature of the movements into the said
calculation.

7. Tracking method according to claim 1, characterized in that at least
one from among the first, second, third, fourth and fifth items of
information furthermore comprises associated error estimations.

8. (canceled)

9. Tracking method according to claim 1, the tracking device comprising a
tracking means comprising means of the filming by the tracking means,
such as adjusting a zoom and/or a focus or an aperture angle,
characterized in that the method furthermore comprises a step of
controlling the adjustment means as a function of the distance of the
moving object or objects with respect to the tracking means.

10. Tracking method according to claim 1, the tracking device comprising
a tracking means comprising means of adjusting the filming by the
tracking means, such as adjusting a zoom and/or a focus or an aperture
angle, characterized in that it comprises a step of controlling the
adjustment means as a function of the uncertainty of the relative
position of the moving object or objects with respect to the tracking
means.

11. Tracking method according to claim 1, characterized in that it also
comprises a step (h) of reception by the tracking device of a request to
track the moving object or objects, then a step of transmission of a
tracking signal attesting the tracking of the said moving object from the
tracking device.

12. Tracking method according to claim 1, characterized in that it
comprises at least two moving objects and a step (j) of reception of a
signal transmitted from one of the moving objects indicating the moving
object to be tracked, then a step (k) of sending a tracking signal
attesting the tracking of the said moving object to be tracked from the
tracking device.

14. Tracking method according to claim 12, characterized in that the
calibration step comprises determining the orientation of a reference
peculiar to the support or to the tracking means, by the intermediary of
accelerometers or inclinometers disposed on the support and determining
the local vertical.

15. Tracking method according to claim 13, characterized in that the
calibration step determining the orientation of a reference peculiar to
the support furthermore comprises a compass which determines the North.

16. Tracking method according claim 1, characterized in that, if the
first and third items of information cannot be defined, the calculation
of the relative position of the moving objector objects with respect to
the tracking means is carried out on the basis of the second, fourth and
fifth items of information.

17. Tracking system for the implementation of the method for tracking a
moving object according to claim 1, characterized in that it comprises a
tracking device comprising at least one tracking means mounted in an
articulated manner on a fixed support, a satellite positioning system and
at least one pressure sensor on the moving object or objects, the
tracking system furthermore comprising a satellite positioning system
mounted on the moving object or objects, a transmitter and a receiver
mounted on the moving object or objects and adapted to transmit and
receive a signal to and from the tracking device.

18. Tracking system for the implementation of the method for tracking a
moving object according to claim 1, characterized in that it comprises a
tracking device comprising at least one tracking means mounted in an
articulated manner on a fixed support, a satellite positioning system and
at least one set of antennas sensitive to direction, the tracking system
furthermore comprising a satellite positioning system mounted on the
moving object or object, a means for measuring atmospheric pressure on
the moving object or objects, a transmitter and a receiver mounted on the
moving object or objects and adapted to transmit and receive a signal to
and from the tracking device.

19. Tracking system according to claim 18, characterized in that the
tracking means is a video camera firmly mounted on a platform that is
articulated on the support.

Description:

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

[0003] Not applicable.

REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC

[0004] Not applicable.

BACKGROUND OF THE INVENTION

[0005] 1. Field of the Invention

[0006] The present invention relates to methods and systems allowing the
tracking of a cooperative moving object by a tracking device, notably
comprising a video camera.

[0007] The present invention applies notably, but not exclusively, to the
tracking of cooperative targets, such as sportspersons in motion, in such
a way that the latter can view their physical and technical performances.

[0008] More particularly, it relates to a method for the tracking of at
least one moving object by a tracking device, the tracking device
comprising at least one tracking means, such as a video camera, mounted
in an articulated manner on an easily transportable, fixed or mobile
support.

[0010] Systems for automatically tracking moving objects by video cameras
are known in the prior art. Notably, systems are known having an
automatically orientable line of sight in order that the latter may track
the moving object, or also video systems comprising automatically
adjusted zoom and/or focus as a function of information gathered by the
system.

[0011] The document US2007/0133979 describes such a system in which
transmitters disposed on the objects to be tracked communicate with a
receiver connected to the video camera and transmit information making it
possible to orient the video camera automatically on the basis of the
intensity of the signal received by directive antennas. However, antennas
of this type have the disadvantage of being bulky and costly for
accurately tracking moving objects from a great distance.

[0012] The document US2010/0208941 also describes a system in which
transmitters disposed on the objects to be tracked communicate with a
receiver connected to the video camera. In this system, the calculation
of the angle of inclination of the video camera is based on satellite
positioning information and/or on radar positioning information.
Altimetric positioning accuracy by satellite is known to be worse than
the estimation of latitude or of longitude. This accuracy is typically of
the order to 100 m whilst the accuracy of the longitude or of the
latitude is of the order of 10 m. Moreover, when the moving object and
the system are close to the ground, radar is particularly disturbed, in
its estimation of the angle of inclination, by reflections of the signal
on the ground and by refraction in the layer of air when the latter
exhibits a temperature gradient.

[0013] In fact, it is known that a signal reflected on the ground or
refracted by the layer of air does not have the same direction of arrival
as the direct signal, which can intertere with the determination of the
direction of arrival of the direct signal. False radar signals created by
these phenomena are referred to as "phantoms" and they are sometimes
difficult to identify and eliminate. When they can be identified and
eliminated, it is at the cost of extremely sophisticated algorithms,
which are expensive to run in real time. The system of the prior art has
the major disadvantage of a complex and inefficient determination of the
angle of inclination.

[0014] In the prior art, tracking a moving object can also be carried out
by means of automatic analysis of the images in order to adjust the
orientation of the video camera. The tracking can also be carried out by
means of the disposition of infra-red transmitters located on the object,
or again by means of presence detectors located in the ground. These
three methods can be combined in order to give the systems the
information necessary for orienting the line of sight of the video camera
and possibly the adjustment of its zoom. However, these systems are
costly, relatively complex to implement and have a limited range, making
the tracking from a great distance of an object that is hardly
recognizable in the image impossible.

[0015] In sports clubs having few financial means and not able to invest
in the existing complex and too expensive technical systems, recording
the performances of sportspersons is carried out either by leaving the
video camera in a fixed setting without the possibility of stopping
filming or of orienting and zooming in order to observe the sportsperson
correctly over a complete course, or by filming the sportsperson with the
help of a person who must operate the video camera. However, in practice,
an instructor cannot simultaneously film the sportspersons and attend to
safety and teaching. It is therefore necessary to call upon a third
person, which represents an additional cost which is rarely possible for
a sports club. Moreover, the intervention of an additional person filming
the sportspersons, results in tracking choices that are different from
those that the instructor would have desired.

[0016] The purpose of the present invention is to alleviate these
disadvantages and to propose a system making it possible to track a
moving object, indoors or outdoors, at a short distance or at a long
distance, in a robust manner with high performance and low cost, and
which is notably suitable for sports instruction applications.

SUMMARY OF THE INVENTION

[0017] For this purpose, the tracking method is characterized in that it
comprises at least the following steps:

[0018] (a) reception by the
tracking device of a signal transmitted by the moving object or objects
and comprising a first item of information of the position of the moving
object or objects determined from information received from a satellite
positioning system, (b) determination a second item of information of the
relative altitude of the moving object or objects with respect to the
tracking device, and the said second item of information is calculated on
the basis of the atmospheric pressures measured at the altitude of the
tracking device and on the moving object or objects, (c) calculation of
the relative position of the moving object or objects with respect to the
tracking means, from the first and second items of information using a
data fusion technique, (d) orientation of the tracking means towards the
relative position calculated in step (c).

[0019] Due to these arrangements, the items of information regarding the
position of the moving object to be tracked are optimized and obtained by
a simple and robust system, without necessitating the use of costly
and/or bulky equipment as is the case for the systems of the prior art.

[0020] The determination of the relative altitude of a moving object with
respect to the tracking means by means of atmospheric pressure
measurements constitutes a particularly advantageous means for such a
system, since the sought accuracy of a few metres is exactly that
achieved by sensors that are very inexpensive (a few euros), very small
(a few cubic millimetres) and very simple to implement on an entirely
standard electronic board. Thus there is avoided the disadvantages of
bulky radar antennas which are expensive and require complex signal
processing, the disadvantages of the altitude delivered by satellite
positioning systems whose accuracy is inadequate for framing a
sportsperson to within a few metres in order to be able to zoom without
losing the image, the disadvantages of image processing techniques where
it can be very complex or even impossible to recognize the sportsperson
to be filmed when that sportsperson is enclosed in a crowd or in a
complex landscape, the disadvantages of the limited range of certain
systems since in this case the pressure can be transmitted by radio at
distances of the order of a kilometre by radio systems.

[0021] According to other features:

[0022] the method also comprises a
step (e) of determination of a third item of information of position of
the tracking device (12) from information received from a satellite
positioning system;

[0023] the method also comprises a step (f) of
determination of a fourth item of information of the distance between the
moving object or objects and the tracking device;

[0024] the method also
comprises a step (g) of determination of a fifth item of information of
the angle of arrival of the signal in a horizontal plane; such an item of
information can be delivered by antennas sensitive to the direction of
arrival of the signal. When the moving object is a short distance away,
typically less than 50 m, this item of information can be more reliable
than that delivered by the satellite positioning system for aiming the
tracking device towards the moving object. For example an error of
5° in the horizontal direction of arrival of the signal coming
from a moving object situated at 30 m from the tracking device
corresponds to an error of 2.6 m in the position of the moving object,
whilst the information given by the satellite positioning system can have
an error of the order of 10 m;

[0025] the method comprises in step (c)
the third and/or the fourth, and/or the fifth item of information;

[0026]
the calculation of the relative position of the moving object or objects
takes into account a behavioural model of the moving object or objects by
adding information on the nature of the movements into the said
calculation. The expression "nature of the movement" is understood to
mean for example a speed and/or a limited acceleration, and/or a minimum
radius of gyration, and/or any other hypothesis;

[0027] at least one from
among the first, second, third, fourth and fifth items of information
furthermore comprises associated error estimations;

[0028] the second
item of information relating to the altitude is calculated from the
estimation of the angle of arrival in a vertical plane. Such an item of
information can be delivered by antennas sensitive to the direction of
arrival of the signal. This item of information can be more accurate than
that delivered by a satellite positioning system. For example an error of
10° in the vertical direction of arrival of the signal from a
moving object situated at 30 m from the tracking device corresponds to an
altitude error of 5.2 m, whilst the altitude error in the item of
information delivered by a satellite positioning system can be of the
order to 100 m.

[0029] the tracking device comprises a tracking means
comprising means of adjusting the filming by the tracking means, such as
adjusting a zoom and/or a focus or an aperture angle, characterized in
that the method furthermore comprises a step of controlling the
adjustment means as a function of the distance of the moving object or
objects with respect to the tracking means;

[0030] the tracking device
comprises a tracking means comprising means of adjusting the filming by
the tracking means, such as adjusting a zoom and/or a focus or an
aperture angle, characterized in that it comprises a step of controlling
the adjustment means as a function of the uncertainty of the relative
position of the moving object or objects with respect to the tracking
means;

[0031] the method also comprises a step (h) of reception by the
tracking device of a request to track the moving object or objects, then
a step (i) of transmission of a tracking signal attesting the tracking of
the said moving object from the tracking device;

[0032] the method
comprises at least two moving objects and a step (j) of reception of a
signal transmitted from one of the moving objects indicating the moving
object to be tracked, then a step (k) of sending a tracking signal
attesting the tracking of the said moving object to be tracked from the
tracking device;

[0033] the method comprises a prior step of calibration
of the tracking device in order to know the orientation of a reference
peculiar to the support and/or to the tracking means with respect to the
local vertical, or within a local geographic reference system: the
expression "local geographic reference system" refers to the reference
constituted by the three axes, North, East and Vertical. The expression
"local vertical" refers to the direction of gravity;

[0034] the
calibration step consists in determining the orientation of a reference
peculiar to the support or to the tracking means, by the intermediary, of
accelerometers or inclinometers disposed on the support and determining
the local vertical. In this way, the calibration step makes it possible
to know the horizontality of a reference peculiar to the support and/or
to the tracking means in the local geometric reference system;

[0035] the
calibration step determining the orientation of a reference peculiar to
the support furthermore comprises a compass which determines the North.
The calibration step thus makes it possible to know completely the
orientation of a reference peculiar to the support and/or to the tracking
means;

[0036] if the first and third items of information cannot be
defined, the calculation of the relative position of the moving object or
objects with respect to the tracking means is carried out on the basis of
the second, fourth and fifth items of information.

[0037] The relative altitude of the tracking device and of the moving
object is obtained in a reliable and simple manner using pressure
sensors. The required accuracy is that much easier to achieve when the
measurement is differential, as in this case, and not absolute. Thus the
subject to be filmed can be framed to within a few metres, which allows a
sufficient zoom for observing the moving object very well.

[0038] The invention also relates to a tracking system for the
implementation of the method for tracking a moving object described
above, noteworthy in that it comprises a tracking device comprising at
least one tracking means mounted in an articulated manner on a fixed
support, a satellite positioning system and at least one pressure sensor
on the moving object or objects, the tracking system furthermore
comprising a satellite positioning system mounted on the moving object or
objects, a transmitter and a receiver mounted on the moving object or
objects and adapted to transmit and receive a signal to and from the
tracking device.

[0039] According to other features, the system comprises a tracking device
comprising at least one tracking means mounted in an articulated manner
on a fixed support, a satellite positioning system and at least one set
of antennas sensitive to direction, the tracking system furthermore
comprising a satellite positioning system mounted on the moving object or
objects, a transmitter and a receiver mounted on the moving object or
objects and adapted to transmit and receive a signal to and from the
tracking device.

[0040] According to other features, the tracking means is a video camera
firmly mounted on a platform that is articulated on the support.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0041] The present invention will be better understood on reading the
following detailed description, given with reference to the appended
drawings in which:

[0042] FIG. 1 is a diagrammatic representation of a set of moving objects
14, one of which is tracked by the tracking device according to the
invention,

[0043] FIG. 2 is a diagrammatic representation of one embodiment of the
method;

[0044] FIG. 3 is a diagrammatic illustration of the components
constituting a moving object 14.

DETAILED DESCRIPTION OF THE INVENTION

[0045] In the following description, the terms "horizontal", "vertical",
"high" and "low" are used according to their usual meaning within the
terrestrial reference system,

[0046] The teaching of certain sports benefits from video when the latter
can be used easily and at affordable cost. Team sports, for example
football, volleyball, etc., use video for the analysis of their play and
that of their opponents. A sport like parachuting fully integrates video
in the sport, using it for the analysis of performance from the very
beginning. The instruction of sportspersons during their training
sessions or their competitions therefore has real benefit, provided that
this recording can be used simply and at affordable cost, which is not
the case for all sports. In certain sports, it is a matter for example of
not having to have a third person dedicated to this function. The present
invention proposes a system having the advantages described above, its
application to sport being only one example.

[0047] More particularly, the invention relates to a tracking system 10
notably constituted by a tracking device 12 adapted to track a moving
object 14 such as a sportsperson. The tracking device 12 comprises a
support 16, such as a tripod, upon which is articulated a tracking means
18, such as a video camera. The video camera 18 is connected to the
support 16 by the intermediary of a platform 20 upon which, it is firmly
mounted. The platform 20 is articulated on the support 16 and extends in
a first plane. A set of motors 22 makes it possible to move the platform
20 with respect to the support 16. The motors 22 allow a movement of the
platform 20 in rotation about a vertical axis so that the video camera 18
can be moved with a horizontal panning motion (PAN) and in rotation about
a horizontal axis so that the video camera 18 can be moved with a
vertical tilting motion (TILT). The servo-controlled platform 20 makes it
possible to achieve all of the angular positions allowing the tracking of
the moving objects 14.

[0048] The tracking device 12 is furthermore equipped with a means 24 of
transmission and reception of a radio or ultrasonic signal. These means
are notably constituted by a set of antennas 24 sensitive to direction,
for example of the Adcock antenna type, which in a particular embodiment
is mounted on the video camera 18 or on the platform 20, in order that
the direction of arrival of signals from the moving object 14 is always
seen at the same angle, thus reducing errors as is known in a zero-error
control system. According to another embodiment, the set of antennas is
mounted on the fixed support 16 in such a way that the external
environment of the system is seen at constant angles in order to limit
interference in the measurements. The device is furthermore provided with
a satellite positioning system 26, such as a GPS. The tracking device 12
furthermore comprises a processing unit 28 which is connected to the set
of antennas 24. The processing unit 28 processes the information relating
to the signal received by the set of antennas 24 and the position
information of the tracking device 12 coming from the satellite
positioning system, as well as the controls for the motors 22, and
possibly any type of interface with the tracking means 18, as well as any
type of interface with the user.

[0049] The tracking system also comprises a transmitter 30a-receiver 30b
assembly disposed on the moving object 14. According to a preferred
variant embodiment, the system comprises several transmitter 30a-receiver
30b assemblies, each assembly being disposed on a moving object 14 to be
tracked. Each moving object 14 is furthermore provided with a satellite
positioning system 32, such as a GPS. The transmitter 30a-receiver 30b
assemblies are adapted to communicate with the tracking device 12 and to
transmit a signal containing at least an identifier specific to each
moving object and a first item of position information of the moving
object 14.

[0050] The position data of the tracking device 12 and the position data
of the moving object 14 have errors notably due to calculation
uncertainties or again to measurement tolerances of the equipment. For
example, the position information gathered from a GPS position comprises
the position data and an accuracy reduction coefficient making it
possible to know if an evaluated GPS position is reliable or not. The
method of the present invention takes account of these errors and the
position of the moving object 14 and the position of the tracking device
12 will be respectively determined by a first and a third item of
position information comprising the position data coming from the
satellite positioning system and the associated error estimations.

[0051] The operation of the system 10 for tracking a moving object 14 by
the tracking device 12 will be described below with reference to FIG. 2.

[0052] Initially, a step of calibration of the system is carried out in
order to know the position of the support in the local geographic
reference system. For this purpose, in a preferred embodiment, the
tracking system 10 according to the invention furthermore comprises a
compass 34 and two inclinometers or accelerometers 36 providing
perpendicular measurements situated on the support 16. Due to these
arrangements, it is possible to determine easily the position of the
support 16 in the local geographic reference system, referenced by the
three axes North, East and Vertical, and then to derive, from knowledge
of the state of the motors, the initial orientation of the tracking means
18 in order to be able to determine in which direction and by how much to
rotate in order to aim at the moving object 14.

[0053] The items of information coming from the compass 34 and from the
inclinometers 36 are taken into account when the tracking device 12 is
placed on the ground and prior to any tracking of a moving object 14. The
calibration step consists in determining the orientation of a first
reference peculiar to the support (16) or to the tracking means (18), by
the intermediary of accelerometers or of inclinometers disposed on the
support or the tracking means (18) and determining the local vertical,
and of a compass which determines the North. The addition of a compass 34
and of inclinometers or accelerometers 36 on the system is inexpensive,
simple and reliable, and allows the portability of the system to
different places without obliging the user to carry out complex
operations in order to recalibrate the system.

[0054] After the calibration step, the tracking method consists in
acquiring, in a synchronized or unsynchronized manner, successively or
not successively, various items of information and in combining them in
order to obtain, at all times an estimation of the direction and of the
distance of the moving object to be tracked, and therefore various
tracking instructions:

[0055] the transmitter 30a disposed on the
moving object 14 transmits a signal several times per second, for example
between 1 and 1000 times per second. The transmission of the signal can
be carried out regularly and automatically, or it can be carried out in
response to an interrogation signal, such as a request from the tracking
device 12. At regular intervals, for example 1 to 100 times per second,
the signal comprises the identification codes of the moving object 14 and
the first item of position information of the moving object 14 in real
time. The signal can moreover comprise an identification code of the
tracking system 10, so that several systems can operate in the same area
without interference.

[0056] the tracking device 12 determines the third
item of position information by the intermediary of the data coming from
the satellite positioning system 26. According to a variant embodiment,
the third item of position information can have been entered manually
through a user interface, or the position can come from an external unit
provided with a (GPS receiver and having been placed in the immediate
proximity of the tracking device 12 and adapted to communicate with it.

[0057] When the signal is received by the set of antennas 24, it is then
analysed by the processing unit 28. If the processing unit 28 detects in
the signal the presence of the identification code of the system and if
necessary that of the moving object to be tracked, then the signal is
taken into account in the updating of the estimations and the first and
third items of information of the GPS position are processed by the
processing unit 28. In one embodiment, the first items of position
information of the moving object 14 are used for calculating, with the
third items of information of the position of the tracking device 12, a
first value of direction of arrival a1, b1 and a first value of distance
D1 between the tracking device 12 and the moving object 14.

[0058] Moreover, the set of antennas 24 connected to the tracking means 18
is sensitive to the direction of arrival of the signal transmitted by the
moving object 14. When the signal is received by the set of antennas 24
and its identification is validated, the processing unit 28 estimates at
least a first angle of arrival of the signal coming from the moving
object 14, in the first plane, in order to derive from it a second value
of direction a2. A second item of information comprising the second
direction of arrival and the associated error estimations is then
determined.

[0059] According to a preferred embodiment, the system comprises two sets
of antennas sensitive to direction and the second direction value is
defined by a pair of angles. The angles can be determined respectively
with respect to the first plane in which the platform 20 extends and with
respect to a second plane perpendicular to the first plane.

[0060] According to a variant of the invention, the tracking system 10
comprises a pressure sensor 42 fixed on the tracking device 12 and a
pressure sensor 44 fixed on the moving object, so that an altitude
difference DZ2 can be calculated, and so that an angle of direction b can
be derived by the processing unit by combination with the distance
information derived from the position information, by means of the
equation: angle b=arcsin (DZ2/distance)

[0061] The processing unit 28 also comprises means allowing it to estimate
a second value of distance D2 of the moving object with respect to the
video camera 18 on the basis of the intensity of the signal received from
the moving object. According to a variant, the second distance value D2
can be estimated on the basis of the forward-and-return time of a
forward-and-return signal between the tracking device 12 and the moving
object. A fourth item of information comprising the second distance value
D2 of the moving object 14 with respect to the tracking device 12 and the
error estimations associated with its calculation is then established.

[0062] The processing unit 28 comprises calculating means adapted to
calculate from the previously determined first, second, third and fourth
items of information, the relative position D, a, b of the moving object.
For this purpose, the calculation is carried out by means of a data
fusion technique. These techniques make it possible to mix items of
information coming from different sources in order to obtain an
estimation and more particularly consist in integrating the multiple
items of information for the purpose of reducing the uncertainty of the
resultant information, as indicated in the article entitled "La fusion de
donnees, du capteur au raisonnement (Data fusion, from sensor to
reasoning)" published in the journal "Traitement du signal 1994--volume
11--n.sup.o6".

[0063] For this purpose, the calculation using the data fusion technique
takes account of the reliability of the data used and the degrees of
uncertainty of the input information. The fusion is based on a
combination of items of information weighted by their respective
uncertainties. Data fusion is a process making it possible to make the
best combination, using existing and known calculation methods, of a
multi-source set of data to provide a resultant item of information of
better quality. It is defined that a data fusion technique consists in
retaining only the data that is known to be the most accurate. For
example, between the data from the precision sensor and the data from a
GPS, the data from the GPS would not be taken into account in order to
have better accuracy.

[0064] By way of simple example, if the measurement uncertainties in a1,
a2, b1, b2, D1 and D2 are expressed in the form of respective standard
deviations σa1, σa2, σb1,
σb2, σD1, σD2, the estimations of a, b,
and D can be obtained thus:

D=σD22/(σD12+σD22)*D1+.s-
igma.D12/(σD12+σD22)*D2;

a=σa22/(σa12+σa22)*a1+.s-
igma.a12/(σa12+σa22)*a2;

b=σb22/(σb12+σb22)*b1+.s-
igma.b12/(σb12+σb22)*b2.

[0065] For example, when the moving object is situated a long distance
from the tracking device 12, a distance of more than 50 metres, the first
values of direction a1, b1 and of distance D1 coming from the satellite
positioning system will be given priority, because the satellite
positioning error is independent of distance. On the other hand, at short
range, that is to say for distances less than 50 m, the second values of
distance D2 and of direction a2, b2 will be given priority, for example
allocated a larger weighting factor. The weighting coefficient is defined
on the basis of the error estimations and of the second distance value.

[0066] Thus, when the first and third items of information are definable,
the processing unit 28 takes advantage of the redundancy between the data
coming from the first and third items of information and the data coming
from the second, and fourth items of information in order to optimize the
calculation of the relative position D, a, b of the moving object 14,
despite the errors particular to each type of calculation or of
measurement.

[0067] The main purpose of using data coming from the satellite
positioning system and that coming from the set of sensitive antennas 24
is to improve the aiming accuracy. In fact, the set of directive antennas
24 informs the system of its orientation with respect to the moving
object but is likely to have errors of several degrees. At a distance of
one hundred metres an error of 10° in the line of sight results in
a deviation of about 17 metres between the moving object aimed at and the
point actually at the centre of the video image. At five hundred metres,
this becomes about 87 metres. Consequently, at long distances, the
information coming from the satellite location data is much more
pertinent than that coming from the antennas 24.

[0068] The calculating means of the processing unit 28 use known signal
processing techniques which make it possible to take advantage of the
redundancy of the items of information relative to direction and to
distance in order to do better than can be done with each of the sources
of information taken separately.

[0069] When the system is used inside a room, for example in a swimming
pool in order to track and film swimmers, the first values of direction
a1, b1 and of distance D1 coming from the satellite positioning systems
cannot be defined. In this case, only the second and fourth items of
information are used in order to determine the relative position D, a, b
of the moving object 14.

[0070] According to a variant of the invention, the calculation of the
relative position of the moving object with respect to the tracking means
takes account of a behavioural model of the moving object; speed and
acceleration are limited for example. The taking into account of this
model, like other items of information, is carried out for example by
Kalman filtering, in order to improve the accuracy of the calculation of
the position of the moving object.

[0071] The expression "behavioural model" refers to all of the items of
physical information that will be added to the measurements in the form
of a mathematical model or models. For example, it will be possible to
add the information that the moving object never exceeds a certain
acceleration and/or a certain speed and/or a deflection angle and/or that
its acceleration averaged over time is zero, etc.

[0072] The relative position of the moving object 14 is expressed with
respect to the position and orientation of the video camera 18. It is
then used for determining the controls or the motors 22 in order to
control the movement of the platform 20 or the purpose of orienting the
video camera 18 towards the moving object 14.

[0073] The video camera 18 comprises means of adjusting the filming, such
as adjusting a zoom and/or a focus or an aperture angle. According to a
variant embodiment, the relative position is used for controlling the
adjustment means as a function of the distance of the moving object 14
with respect to the tracking means 18. Pro-vision can also be made for
the method according to the invention to furthermore comprise a step of
controlling the adjustment means as a function of the uncertainty of the
relative position of the moving object with respect to the tracking
means, in such a way as to modify the optical field of the tracking means
as a function of the uncertainty. For example, this control makes it
possible to widen the field when the uncertainty of the relative position
is large and to reduce the field when the uncertainty of the relative
position is relatively small.

[0074] The invention described above was illustrated on the basis of a
tracking system comprising a tracking device 12 and one moving object 14.
The invention operates similarly with several moving objects 14. In this
case, it is then necessary to manage the exchanges between the tracking
device 12 and the multitude of moving objects 14. The continuation of the
description describes, in a non-exhaustive manner, several examples of
management of exchanges between the tracking device 12 and the moving
objects 14. The tracking method is in no way limited to the examples
described below.

[0075] According to a first embodiment, the system comprises a main moving
object 14 and secondary moving objects 14.

[0076] According to a first variant, the main moving object 14 comprises a
designation means adapted to send a signal to the tracking device 12 in
order to designate the moving object 14 to be tracked. This designation
can be activated by a remote control function, for example by pressing a
button 38. The secondary moving objects 14 do not have this operational
feature. The main moving object 14 can designate itself to be tracked.
When the device receives such a designation signal, it transmits a
tracking signal in order to inform the moving object to be tracked and
the main moving object and, if necessary, so that the transmitter 30a can
transmit a signal comprising the information necessary for the
implementation of the tracking method described in the first part of the
description.

[0077] According to a second variant of the invention, the main moving
object 14 sends a signal to the tracking device 12, indicating that the
latter must automatically respond to requests from the secondary moving
objects 14. Thus, when the transmitter 30a of one of the moving objects
14 sends a tracking request, the tracking device 12 receiving the signal
transmits, in response, a tracking signal to the said requesting moving
object 14, and the tracking method described in the first part of the
description is applied to the said moving object 14.

[0078] The moving objects 14 communicate with the tracking device 12 by
communication techniques known to those skilled in the art, notably by a
radio or ultrasonic link. The tracking request from a secondary moving
object 14 can be activated by pressing a button 38 or by voice command or
by any other means.

[0079] According to a variant embodiment, the tracking of the requesting
moving object 14 is carried out during a specified period of time, for
example one minute. During this period of time, the possible tracking
requests from the other secondary moving objects 14 are recorded without
any particular action. At the end of the period of tracking a requesting
moving object 14, the tracking device 12 is controlled in such a way as
to monitor another moving object, preferably the one indicated by the
last tracking request received.

[0080] Provision can also be made so that, in the absence of another
tracking request, the moving object 14 is tracked beyond the specified
time period.

[0081] Preferably, the moving objects 14 have knowledge of the information
relating to their tracking, for example by the switching on or off of an
LED 40 disposed on the moving object 14. The main unit can furthermore
have knowledge of the tracked moving object 14.

[0082] The invention is in no way limited to the embodiments and variant
embodiments described above, which have been given only by way of
example. It can notably be applied to taking photographs using
photographic cameras and to lighting systems that have to track a moving
target in a theatrical scene.